Unreal Tourn. 2004 1player lag

[nlawalker]

I'm having the strangest problem with Unreal Tournament 2004. When playing in single player, I experience terrible control lag. My framerates are fine, and even when tested using the unreal benchmark they are great. If I spectate a game, I can zip around and make lots of fast motions and there's not the slightest glitch. However, when I play, the game frequently things I'm still holding a button I have let go of, often for a second or more. It's making the game almost unplayable.

The REALLY weird thing is that this didn't occur in the demo, which I still have and even tried again WITH the full game installed on the same HD. Again, the demo does not exhibit this problem, but the full game does, even when using the exact same settings/characters/maps.

What could possibly be the problem? I have the game fully patched.